A brief Long Term Evolution (LTE) is a mobile communication system evolved from Universal Mobile Telecommunications System (UMTS), standardized by an international standardization organization, the 3rd Generation Partnership Project (3GPP). The configuration of an LTE system is illustrated in FIG. 1.
FIG. 1 is a view referred to for describing the configuration of an LTE system.
The LTE system may be divided largely into an Evolved UMTS Terrestrial Radio Access Network (E-UTRA) and an Evolved Packet Core (EPC). The E-UTRAN includes UEs and evolved Node Bs (eNBs). A UE is connected to an eNB via a Uu interface and one eNB is connected to another eNB via an X2 interface. The EPC includes a Mobility Management Entity (MME) responsible for control-plane functions and a Serving GateWay (S-GW) responsible for user-plane functions. An eNB is connected to the MME via an S1-MME interface and an eNB is connected to the S-GW via an S1-U interface. These two interfaces are collectively called an S1 interface.
For the Uu interface being an air interface, a radio interface protocol is defined. The radio interface protocol horizontally includes a physical layer, a data link layer, and a network layer and vertically includes a user-plane (U-plane) for user data transmission and a control-plane (C-plane) for control signaling. Based on the lowest three layers of the Open System Interconnection (OSI) reference model, this radio interface protocol can be divided into Layer 1 (L1) including a physical layer PHY, Layer 2 (L2) including a Medium Access Control/Radio Link Control/Packet Data Convergence Protocol (MAC/RLC/PDCP) layer, and Layer 3 (L3) including a Radio Resource Control (RRC) layer. These layers are defined in pairs between a UE and an E-UTRAN, for data transmission via the Uu interface.
Now a description will be given below of a Long-Term Evolution Advanced (LTE-A) system.
LTE-A is a system developed from LTE to meet 4th Generation mobile communication requirements, that is, IMT-Advanced requirements recommended by the International Telecommunication Union-Radio communication sector (ITU-R). The 3GPP which developed the LTE system standard is now actively working on standardization of the LTE-A system.
Major technologies added to the LTE-A system are carrier aggregation for extending a used bandwidth and flexibly using the bandwidth and use of relays for improving coverage, supporting group mobility and enabling user-centered network deployment.
FIGS. 2 and 3 are views referred to for describing radio protocol layers.
At L1, the PHY layer provides information transfer service to a higher layer on physical channels. The PHY layer is connected to the MAC layer through transport channels and data is transferred between the MAC layer and the PHY layer on the transport channels. The transport channels are largely divided into dedicated transport channels and common transport channels depending on whether the transport channels are shared or not. Data is transmitted on physical channels using radio resources between different PHY layers, that is, the PHY layers of a transmitter and a receiver.
There is a plurality of layers at L2. The MAC layer maps logical channels to transport channels and performs logical channel multiplexing by mapping a plurality of logical channels to one transport channel. The MAC layer is connected to a higher layer, namely the RLC layer, through logical channels. Depending on the types of information carried on the logical channels, the logical channels are classified into control channels that deliver C-plane information and traffic channels that deliver U-plane information.
The RLC layer at L2 adjusts a data size to be suitable for data transmission in the air interface from a lower layer by segmenting and concatenating data received from a higher layer. In order to guarantee various Quality of Service (QoS) requirements of each Radio Bearer (RB), the RLC layer provides three operation modes, Transparent Mode (TM), Unacknowledged Mode (UM), and Acknowledged Mode (AM). Especially, an AM RLC performs a retransmission function through Automatic Repeat and Request (ARQ), for reliable data transmission.
The PDCP layer at L2 compresses a header to reduce the size of an Internet Protocol (IP) packet header of a relatively large size containing unnecessary control information to efficiently transmit an IP packet such as an IPv4 or IPv6 packet via a radio link having a narrow bandwidth. The header compression function enables transmission of necessary information in a header only, thereby increasing the transmission efficiency of a radio link. In addition, the PDCH layer performs a security function in the LTE system. This security function involves ciphering for preventing a third party from eavesdropping and integrity protection for preventing a third party from maliciously modifying data.
The RRC layer at the highest of L3 is defined only in the C-plane. The RRC layer takes charge of controlling logical channels, transport channels, and physical channels in relation to configuration, reconfiguration, and release of Radio Bearers (RBs). An RB is a logical path provided by L1 and L2 in the radio protocol architecture, for data transmission between a UE and a UTRAN. In general, configuring an RB means defining the features of a radio protocol layer and channels needed to provide a specific service and setting specific parameters and an operation scheme. RBs are classified into a Signaling RB (SRB) and Data RB (DRB). The SRB is used as a path via which an RRC message is transmitted on the C-plane and the DRB is used as a path in which user data is transmitted on the U-plane.
Now a description will be given of Carrier Aggregation (CA) in the LTE-A system.
FIG. 4 is a view referred to for describing CA.
As described above, the LTE-A standard is designed as an IMT-Advanced candidate technology of the ITU to satisfy IMT-Advanced technical requirements. Accordingly, extension of a bandwidth from the legacy LTE system is under discussion to satisfy IMT-Advanced technical requirements. For bandwidth extension, carriers available to the legacy LTE system are defined as Component Carriers (CCs) in the LTE-A system. Aggregation of up to 5 CCs is under discussion, as illustrated in FIG. 4. Because a CC may occupy up to 20 MHz as in the LTE system, the CA technology of the LTE-A standard is a concept of extending a bandwidth to up to 100 MHz. The technology of aggregating a plurality of CCs is called CA.
Hereinbelow, a random access procedure performed in the LTE system will be described in greater detail.
In the LTE system, a UE may perform the random access procedure,
when the UE initially accesses an eNB without an RRC connection having been established therebetween,
when the UE initially accesses a target cell during handover,
when the random access procedure is requested by a command from the eNB,
upon generation of uplink data in a situation in which uplink time synchronization has not been acquired or specified radio resources for use in requesting radio resources have not been allocated, or
when radio link failure or handover failure is recovered.
Based on the above description, a general contention-based random access procedure will be described below.
FIG. 5 is a diagram illustrating a signal flow for operations of a UE and an eNB in a contention-based random access procedure.
(1) Transmission of First Message
The UE may select a random access preamble randomly from a random preamble set indicated by system information or a handover command, select Physical Random Access CHannel (PRACH) resources, and transmit the random access preamble through selected Physical PRACH (PRACH) resources (S501).
(2) Reception of Second Message
After transmitting the random access preamble in step S501, the UE attempts to receive a random access response within a random access response reception window indicated through the system information or the handover command by the eNB (S502). To be more specific, the random access response may be transmitted in the form of a Medium Access Control Protocol Data Unit (MAC PDU) and the MAC PDU may be delivered over a Physical Downlink Shared CHannel (PDSCH). To receive information on the PDSCH successfully, the UE preferably monitors a Physical Downlink Control CHannel (PDCCH). That is, the PDCCH preferably carries information about a UE to receive the PDSCH, information about the frequency and time of radio resources of the PDSCH, and information about the transmission format of the PDSCH. Once the UE succeeds in receiving the PDCCH destined therefor, the UE may successfully receive a random access response over the PDSCH according to information carried over the PDCCH. The random access response may include an identifier (ID) of the random access preamble (e.g. a Random Access Preamble ID (RAPID)), an Uplink (UL) Grant indicating uplink radio resources, a temporary Cell-Radio Network Temporary Identify (C-RNTI), and a Timing Advance Command (TAC).
The reason for including the RAPID in the random access response is that because one random access response may contain random access response information for one or more UEs, it is necessary to indicate a UE to which the UL Grant, the temporary C-RNTI, and the TAC are valid. It is assumed in step S502 that the ID of the random access preamble is identical to the RAPID included in the random access response. Thus, the UE may receive the UL Grant, the temporary C-RNTI, and the TAC.
(3) Transmission of Third Message
Upon receipt of a valid random access response, the UE processes information included in the random access response. That is, the UE applies the TAC and stores the temporary C-RNTI. In addition, the UE may store data to be transmitted in a message3 buffer in correspondence with the reception of a valid random access response.
Meanwhile, the UE transmits data (i.e. a third message) to the eNB using the received UL Grant (S503). The third message should include an ID of the UE. In the contention-based random access procedure, the eNB cannot identify UEs that perform the random access procedure. However, the eNB should identify the UEs to avoid later-collision among them.
Two methods have been discussed to include the ID of the UE in the third message. One of the methods is that if the UE has a valid C-RNTI allocated by the cell before the random access procedure, the UE transmits its C-RNTI in an uplink signal corresponding to the UL Grant. On the other hand, if a valid C-RNTI has not been allocated to the UE before the random access procedure, the UE transmits its UE ID (e.g. S-TMSI or a random ID) in data. In general, the UE ID is longer than the C-RNTI. If the UE transmits data corresponding to the UL Grant, the UE activates a Contention Resolution (CR) timer to avoid contention.
(4) Reception of Fourth Message
After transmitting its ID in data according to the UL Grant included in the random access response, the UE awaits reception of a command for contention resolution from the eNB. That is, the UE attempts to receive a PDCCH in order to receive a specific message (S504). For PDCCH reception, two methods may be considered. When the third message is transmitted using the C-RNTI according to the UL Grant as described above, the UE attempts to receive a PDCCH using the C-RNTI. If the ID included in the third message is the UE ID, the UE may attempt to receive a PDCCH using the temporary C-RNTI included in the random access preamble. In the former case, if the UE receives a PDCCH using the C-RNTI before expiration of the CR timer, the UE ends the random access procedure, determining that the random access procedure has been performed normally. In the latter case, if the UE receives a PDCCH using the temporary C-RNTI before expiration of the CR timer, the UE checks data received on a PDSCH indicated by the PDCCH. If the data includes its UE ID, the UE ends the random access procedure, determining that the random access procedure has been performed normally.
Meanwhile, a contention free random access procedure is ended only by transmitting first and second messages, which is different from the contention-based random access procedure shown in FIG. 5. However, the UE is allocated a random access preamble by the eNB before it transmits a random access preamble as the first message to the eNB. The UE transmits the allocated random access preamble as the first message to the eNB, and ends the random access procedure by receiving a random access response from the eNB.